Flexible membrane diffusers have been used in the diffusion of gases into liquids, one example being the aeration of wastewater. The flexible membranes have been used with tubular and disc type diffusers. Exemplary of a tubular membrane diffuser is U.S. Pat. No. 4,960,546 to Tharp.
Flexible membrane diffusers are conventionally constructed of rubber or a similar material which is punctured to provide a large number of perforations. When gas is applied to the diffuser, the gas pressure expands the membrane away from the diffuser body and causes the perforations to open so that the gas discharges through them in the form of fine bubbles, which transfer gas efficiently to the liquid. When the gas pressure is relieved, the membrane collapses on the diffuser body to close the perforations and prevent the liquid from entering the diffuser.
Although flexible membrane diffusers are advantageous in many respects and have achieved widespread acceptance in a variety of gas diffusion applications, they are not wholly free of problems. Rubber or synthetic rubber is typically used to construct the membrane. In a wastewater treatment application and in other applications, materials in the liquid can become deposited on and build up on the membrane to clog or partially clog the perforations and thus reduce the efficiency of the diffuser. For example, fats, grease and other substances which are commonly found in wastewater can adhere to the membrane. Calcium and calcium compounds such as calcium carbonate and calcium sulfate as well as other substances are especially problematic when they precipitate and build up on the diffuser membrane. Biological growth can also build up and compromise the diffuser efficiency.
Diffuser membranes can also be chemically degraded by solvents and various other types of chemicals that may be present in the liquid. This chemical degradation combined with the repeated expansion and contraction of the membrane can weaken the membrane and cause premature structural failure.
Pending U.S. Patent Application published under publication number U.S. 2005/00324 A1 discloses a multiple layer article which is indicated as being useful as a diffuser membrane. A fluoroelastomer film is applied to a partially cured substrate, and the film and the substrate are thereafter cured together to complete the construction of the membrane.
This multiple step process is disadvantageous in a number of respects. First, it is complicated and costly. Also, because the substrate is only partially cured in the initial step and the film has been applied before the substrate and film are heated together, it is not possible to optimize both the curing of the rubber substrate and the bonding of the fluoroelastomer film to the substrate. The time and temperature requirements for curing differ from those necessary to achieve optimal bonding between the two materials, so either the curing or the bonding must necessarily be compromised. The result is a product that has either an inadequately cured substrate or an inadequate bond between the layers.
Another problem is that there is no opportunity to clean the substrate because the curing process is interrupted and is only partially completed at the time the film is applied at an elevated temperature. If contaminants are present, they cannot be removed by solvents or other cleaning processes and can interfere with the bonding to the point of destroying any ability to properly bond the materials together. The fluoroelastomer layer must be applied as a film, thus making it impossible to apply the coating layer in other manners such as spraying, brushing, rolling, electrostatic application or other application techniques which may be preferable in some cases. For similar reasons, the ability to vary the coating thickness is limited.
Another problem with this process is that the substrate remains uncoated within the perforations and is subject to biological buildup at those locations. If biological growth accumulates on the edges of the perforations, severe clogging can occur and significantly reduce the membrane efficiency, a decided possibility when the environment is difficult from a biological standpoint.